(a) Technical Field of the Invention
The present invention is generally related to implantable medical devices, and more particular to an implantable medical device and system for implantation inside human bodies capable of wireless charging.                (b) Description of the Prior Art        
Through the advancement of technology, medical devices have already been miniaturized so that they can be implanted inside human bodies. The clinical applications of these implantable medical devices have been gaining widespread acceptance. Active implantable medical devices such as implantable nerve stimulation devices, glucose sensors, pacemakers, etc., all require a power source where battery is a common solution. However, the lifetime of the implantable medical device depends on the capacity of the one-time battery.
Recent development has chosen to transmit power through wireless means. Wireless power suffers inferior efficiency compared to wired power. However, this is a valuable technique for implantable medical devices so that a patient is relieved from the suffering of having a surgery simply because the implantable medical device has run out of power or the battery is out of order. The operation life of the implantable medical device is also significantly enhanced.
Resonant inductive coupling is generally employed in the wireless transmission of electrical power. The transmission end and the reception end all have LC resonance mechanisms so that the transmission end is easier in producing high-power time-varying current, corresponding magnetic fluxes are induced on the inductive coils, and the reception end is easier in obtaining the transmitted energy.
When this technique is applied to the implantable medical devices, usually the reception coil has to be aligned with the transmission coil in an external control device so as to obtain the highest amount of energy. If the transmission and reception coils are perpendicular, a less amount of energy is received and the implantable medical device may not be able to function. On the other hand, the depth of the implantable medical device inside human body also affects the distance between the transmission and reception coils. Therefore, when a fixed amount of power is output from the external control device, the implantable medical device may obtain different amounts of power due to the distance and alignment between the transmission and reception coils. The received power may be too high so that the implantable medical device's temperature rises, or it exceeds the limitation of electronic components and damages the implantable medical device. The power may also be too small so that the implantable medical device cannot function correctly. The control of the transmission power is therefore an important issue.